1. Field of the Invention
The invention relates generally to diagonally fed electric microstrip dipole antennas and specifically to such antennas where the feedpoint and via inductance are used to provide a low noise amplifier with the optimum impedance for minimum noise figure (.GAMMA.b).
2. Description of the Prior Art
FIG. 1 shows a conventional system 10 with a Global Positioning System (GPS) antenna 12, a cable 14, and a low-noise amplifier (LNA) 16 in a typical configuration where the coaxial cable 14 (with a loss equal to Lc) links microstrip antenna 12 (with a feedpoint and via inductance having an optimum noise figure source impedance (.GAMMA.s) equal to zero) to the LNA 16 having an input loss (Linput of Lc+L.sub.imn). A fifty ohm match is made possible by an input matching network (IMN) 18 which drives an active device 20 and an output match network (OMN) 22. The noise figure (NF) contribution of the cable and IMN 18 is given by the available gain of the cable 14 and IMW 18.
The input loss of the cable 14 and IMN 18 add directly to the NF of the system F.sub.sys. ##EQU1## where F.sub.ROA =NF of the rest Of the GPS receiver following LNA 16.
As the present invention does, incorporating LNA 16 in antenna 12 and repositioning cable 14 will reduce the system NF by the amount of input loss, if G.sub.LNA &gt;&gt;L.sub.c L.sub.IMN, ##EQU2## The NF reduction is then, EQU F.sub.sys -F'.sub.sys =L.sub.c L.sub.IMN. (4)
A diagonally fed electric microstrip antenna is described by Kaloi in U.S. Pat. No. 3,984,834, issued Oct. 5, 1976. The feedpoint is located along a diagonal with respect to the rectangular antenna element on a dielectric substrate. This particular point of feed is said by Kaloi to cause the antenna to operate in a degenerate mode where two oscillations occur at the same frequency. These oscillations occur along the X axis and the Y axis. The respective axis dimensions determine the resonant frequencies of each. Design equations for this type of antenna are presented by Kaloi, and so are not repeated here.
The feedpoint of a diagonally fed electric microstrip antenna can be chosen to present a particular impedance, for example fifty ohms. However, such a point may not be the point that produces a minimum of noise. Therefore, a diagonally fed electric microstrip antenna with a minimum noise feedpoint and an integrated LNA is needed to permit the reliable operation of high performance systems, such as personal, hand-held GPS receivers.